scripts: fix dumping execution backtrace

Dumping the backtrace has been broken since changeset #652e56d6d35a:
scripts: execute each steps in a subshell

We can spawn sub-sub-shells in some cases.

The way the fault handler works is to dump the backtrace, but to avoid
printing it once for every sub-shell (which could get quite confusing),
it simply exits when it detects that it is being run in a sub-shell,
leaving to the top-level shell the work to dump the backtrace.

Because each step is executed in its own sub-shell, the variable arrays
that contain the step name, the source file and line number, are lost
when exiting the per-step sub-shell.

Hence, the backtrace is currently limited to printing only the top-level
main procedure of the shell.

Fix this thus:
- when dumping the bckatraces for the steps & the functions, remember
it was dumped, and only dump it if it was not already dumped
- at the top-level shell, print the hints

Also, rename the top-level step label.

Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 4193d6e6a17430a177fa88c287879c2c35e319f3)

 diff -durN gcc-3.4.6.orig/gcc/config/sh/lib1funcs.asm gcc-3.4.6/gcc/config/sh/lib1funcs.asm

 --- gcc-3.4.6.orig/gcc/config/sh/lib1funcs.asm	2003-08-12 03:25:07.000000000 +0200

 +++ gcc-3.4.6/gcc/config/sh/lib1funcs.asm	2007-08-15 23:04:26.000000000 +0200

 @@ -37,9 +37,19 @@

     ELF local label prefixes by J"orn Rennecke

     amylaar@cygnus.com  */

 +#define ALIAS(X,Y)	.global GLOBAL(X); .set GLOBAL(X),GLOBAL(Y)

 +

  #ifdef __ELF__

  #define LOCAL(X)	.L_##X

 -#define FUNC(X)		.type X,@function

 +

 +#if 1 /* ??? The export list mechanism is broken, everything that is not

 +	 hidden is exported.  */

 +#undef FUNC

 +#define FUNC(X)		.type X,@function; .hidden X

 +#undef ALIAS

 +#define ALIAS(X,Y)	.global GLOBAL(X); .set GLOBAL(X),GLOBAL(Y); .hidden GLOBAL(X)

 +#endif

 +

  #define ENDFUNC0(X)	.Lfe_##X: .size X,.Lfe_##X-X

  #define ENDFUNC(X)	ENDFUNC0(X)

  #else